1. Field of the Invention
This invention relates to a radiation detecting system suitable for applying to a radiation image taking system such as an X-ray system, and more particularly to a direct-conversion type radiation detecting system.
2. Description of the Related Art
In the medical radiation image taking, there has been known a radiation detecting system using a radiation sensor (having semiconductor as a main component) which detects radiation and converts it to an electric signal. As the radiation sensor, there has been known those of a direct conversion system where radiation is directly converted to electric charges and the electric charges are stored and an indirect conversion system where radiation is once converted to light by a scintillator or the like, the light is converted to electric charges by a photoconductive layer and the electric charges are stored. From the reading system, the radiation sensor is broadly divided into the following two systems. That is, one is optical reading system, where the image is read out by a radiation sensor which generates electric charges by the use of a semiconductor in response to projection of light and the other is a so-called TFT system, where the image is read out by storing electric charges generated in response to projection of radiation and reading the stored electric charges by turning on and off electric switches each for one pixel such as thin film transistors (TFT).
As a radiation sensor of an indirect conversion system using CsI:Na as the scintillator and using a-Se as the photoconductive layer, there is disclosed in “New CsI:Na-Selenium X-ray Detector”, D. A. Seok et al., Proceedings of SPIE, Vol. 5368, pp. 633-638, 2004 a radiation sensor where Parylene film 1 to 9 μm thick as a dielectric layer is provided on a-Se as the photoconductive layer. Further, in U.S. Pat. No. 5,880,472, there is disclosed a radiation sensor having a dielectric layer film of PC, polyester, Parylene or the like 30 to 150 μm thick.
On the other hand, the direct conversion type radiation sensor detects the radiation by imparting a predetermined bias voltage to a voltage imparting electrode formed on the surface of a radiation-sensitive semiconductor film (recording photoconductive layer) and taking out a radiation detecting signal by collecting carriers generated in response to projection of radiation with a carrier collecting electrode formed on the back side of semiconductor film and the recording photoconductive layer is often formed by amorphous selenium (a-se) since it exhibits high dark resistance and is excellent in response speed.
Further, the direct conversion type radiation sensor is often provided with a carrier selective semi-insulating layer in order to give rectifying characteristics to the interface of the voltage imparting electrode and the photoconductive layer and to reduce the dark current. For example, there is disclosed Sb2S3 as the carrier selective semi-insulating layer in U.S. Pat. No. 6,495,817.
However, in the radiation sensor disclosed in “New CsI:Na-Selenium X-ray Detector”, D. A. Seok et al., Proceedings of SPIE, Vol. 5368, pp. 633-638, 2004 or U.S. Pat. No. 5,880,472 the dielectric layer is relatively thick and whether the dark current characteristics and the sensitivity are compatible each other is unknown.
Further, there is a problem that, when the carrier selective semi-insulating layer is formed over the recording photoconductive layer of a-Se by deposition, the a-Se layer is crystallized under the heat of the deposition, which leads to generation of image defect, generation of noise, and deterioration of S/N ratio. Further, when Sb2S3 layer is formed as the carrier selective semi-insulating layer, the large difference in the thermal expansion coefficient between the carrier selective semi-insulating layer and a-Se layer can make cracks in the Sb2S3 layer due to change in the environmental temperature and warpage of the substrate.